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Current-shear modification of the fetch limited growth of wave energy.

by

Haus, Brian K.1, Rafael J. Ramos1, Thomas M. Cook 2, Lynn K. Shay1, Jorge Martinez-Pedraja1
1 Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, bhaus@rsmas.miami.edu, nshay@rsmas.miami.edu, rramos@rsmas.miami.edu, jmartinez@rsmas.miami.edu
2 Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, tcook@ucsd.edu

Abstract

Two High Frequency Wellen Radars (WERAs) have been operating in Southeast Florida since June 2004 as a part of the Southeast Atlantic Coastal Ocean Observing System (SEACOOS). The 16 MHz systems illuminate a large area of the Florida Straits, which is dominated by the highly variable Florida Current system. Typically a continuous wave is transmitted, which allows surface radar backscatter data to be acquired at ranges up to 110 km on a 1.2-km grid every 10 minutes. The backscattered Doppler spectrum is then processed at 5000 locations for each sample to provide real-time near-surface current measurements. An on-offshore transect of these spectra with samples every 1.2 km, from 3-30 km offshore is then processed to extract significant wave height (Hs) estimates using a modified version of the method developed by Barrick (1977).

The calibration coefficients required for this empirical approach were derived from comparison with in-situ measurements conducted during the SEACOOS supported mini-waves experiment conducted in spring 2005. The coefficients were derived from the best fit between the WERA observed 2nd/1st order backscatter and the Hs from a Sontek waves ADP collected for a 25 day period. The results were validated by comparison with Hs observed by a Tri-Axys buoy in the same area but over a succeeding 50 day period. The correlation between all of the HF radar cells and the buoy revealed that the radar observed Hs compared well within 45° of the boresight from each site, but dropped to low values at large angles. By averaging information from both sites the rms difference between the radar observations and the buoy decreased considerably. This is likely due to the directional spreading of the surface waves relative to the radar look direction.

Selecting periods when the wind and radar boresight were aligned, the fetch-limited wave growth when the winds are directed offshore was computed over a range of non-dimensional fetches. The growth of wave energy compares well with previous empirical formulations, where the scatter is of the same order as in-situ wave observations. However, the wave energy deviates significantly from the empirical formulations as the waves propagate across the high current shear region along the western boundary of the Florida Current due to wave-current interactions.

 
 
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